Gold is widely dispersed throughout the earth's crust. Naturally occurring metallic, or native, gold usually contains variable amounts of silver, copper, platinum, palladium, or certain other elements mixed with it. Gold is typically recovered by placer mining of alluvial deposits, by load or vein mining, and as a by-product of base-metal mining.
Placer mining, the oldest method, entails exploiting the high density of gold to separate it from the much lighter siliceous material with which it is found. The alluvial deposits mined by placer methods are the gold-bearing sands and gravel that have been deposited by rapidly moving streams and rivers at places where they widen or for some other reason lose speed. As the current slows, the sediment being carried downstream settles to the bottom.
Silver, unlike gold, is found in many naturally occurring minerals. However, most of the silver (as well as gold) produced today is recovered as a by-product of the treatment of copper, lead and zinc ores from load mining. Tremendous tonages of gold and silver containing ore are treated throughout the world since most ores contain an extremely low percentage of gold and silver. For example, as much as ten to twenty tons of ore might be processed for every ounce of silver and gold obtained.
The cyanide process, introduced in the late 1800's, is still generally used for recovering gold and silver from ores. With such process, the ore is pulverized and leached with a dilute sodium cyanide water solution. The silver and gold form water soluble sodium-silver-and sodium-gold complexes. The mixture is held for several hours in large tanks equipped with agitators. The finished mixture is filtered and the solids discarded. The filtrate is generally treated with finely divided zinc dust that causes the silver and gold to precipitate from the solution. This precipitate is filtered off, silver and gold separated, melted, and cast into bullion bars.
The use of cyanide leaching is typically considered as being primarily useful on oxide type (nonsulfide containing) ores. Sulfide ores tend to give up their values very reluctantly and at higher costs. Where sulfides are present, they are typically removed from ores by roasting (smelting). However, dealing with the pollution can be very expensive. Environmental Protection Agency regulations in some cases have become so stringent that costs for compliance to minimize pollution have made many deposits uneconomical.
Further, cyanide is a dangerous substance that when not properly used is a threat to human life. Safe use of cyanide in the gold and silver recovery processes requires maintenance of a pH of at least 101/2. Cyanide at lower pH's releases cyanide gas which can be fatal when coming into contact with humans.
Carbonaceous ores typically contain graphitic or activated carbon, and long chain organic compounds similar to humic acids. Adsorption of gold or gold cyanide complexes onto the carbonaceous material interferes with the recovery of gold from carbonaceous ores. As such, carbonaceous ores are not readily treatable with cyanide until the carbon is removed, such as by roasting.
Non-cyanide leaching agents and processes have been developed, but are not widely used for gold and silver recovery at this time. For example, thiourea has been demonstrated to be a successful leachant for the recovery of precious metals. However, to be useful the reactant mixture must be kept at a low pH of around 1.4. At high pH, thiourea is rapidly oxidized to sulphur. Further, gold and silver thiourea complexes are cationic and are strongly absorbed into clay minerals. These factors and the high cost of thiourea have prevented its use in commercial production.
It would be desirable to develop improved processes for recovering gold and silver that do not require the use of cyanide. Such processes would also preferably eliminate substantially all of the problems regarding air, water and soil pollution, and at the same time effectively function to recover gold and silver at a low cost so as to be competitive in the marketplace.